The non-aqueous electrolyte secondary batteries include a positive electrode, a negative electrode, a separator interposed between the positive and negative electrodes, and a non-aqueous electrolyte. In recent years, in the non-aqueous electrolyte secondary batteries that have been put into practical use, a microporous film mainly composed of polyolefin is used as the separator, and a non-aqueous electrolyte prepared by dissolving a lithium salt such as LiBF4 and LiPF6 in a nonprotic non-aqueous solvent is generally used as the non-aqueous electrolyte. As a positive electrode active material, lithium cobalt oxides (e.g. LiCoO2) that have a high electric potential versus lithium and a superior safety, and that can be relatively readily synthesized are used; as a negative electrode active material, a variety of carbon materials such as graphite are used.
Under such circumstances, regarding the positive electrode active material, from the viewpoint of realizing a higher capacity, attempts have been made actively to put lithium nickel oxides (e.g. LiNiO2) into practical use. In particular, lithium nickel oxides are expected to be developed into uses requiring durability for a long period of time and reliability.
However, the non-aqueous electrolyte secondary batteries using conventional lithium nickel oxides as the positive electrode active material have a problem, for example, of a decrease of output characteristics owing to an increase of an internal impedance. Suppression of the increase of the internal impedance is a particularly important problem in uses requiring a high output.
Therefore, with the aim of improving output characteristics, Patent Literature 1 proposes composite oxides of lithium-nickel-manganese-cobalt based having a volume resistivity of 5×105 Ω·cm or less when compressed at a pressure of 40 MPa, and a ratio (C/S value) of concentration of contained carbon C (% by weight) relative to BET specific surface area S (m2/g) of 0.25 or less. In this literature, first a nickel compound, a manganese compound, and a cobalt compound are pulverized in a liquid medium into an average particle diameter of 0.3 μm, thereby to prepare an uniformly dispersed slurry. Next, this slurry is sprayed and dried to form secondary particles in which primary particles are aggregated. Thereafter, the secondary particles and a lithium compound are sufficiently mixed and the obtained mixture is baked in an atmosphere of a gas containing oxygen.